FirstGroup boss Tim O'Toole has admitted that the implications of a eurozone break-up are so large that companies will struggle to mitigate the consequences.

"It is almost beyond my ken to analyse the impact of such massive events that other people [ie financial experts] can barely understand," the head of the public transport company said.

O'Toole, whose UK businesses include the First Great Western rail franchise and bus networks in Glasgow and Manchester, expressed concerns about a worsening of the crisis and how to cope with the consequences. Those concerns were reflected in FirstGroup's assessment of the British bus market, published in first-half results today, which warned of "challenging" trading conditions due to a faltering economic performance in urban areas.

"We are worried about the UK economy. It is one of the reasons why we put out a cautionary statement with our bus results," said O'Toole. FirstGroup's bus business is being affected by the same factors that are hitting the wider UK economy, with concerns over consumer demand augmented by a cutback in government spending, including a reduction in fuel subsidies for bus operators.

FirstGroup has mitigated the effect of a softening economy with reductions in bus mileage, but O'Toole indicated that an escalation in the eurozone crisis could be too much for any company to handle. "You have to be concerned. If any of these tectonic changes make it worse, that is obviously a concern. It is almost in a category where it is so large there is no management change you can make to accommodate it, so I just continue on my way."

O'Toole spoke as FirstGroup posted a 3% increase in revenues for the six months to 30 September, to nearly £3.2bn, with its rail businesses still performing well. "We continue to see this extremely strong performance in rail," said O'Toole, referring to a 9% increase in fare revenues at its First Great Western, First Capital Connect, First TransPennine, ScotRail and Hull Trains franchises. FirstGroup's troubled US school bus business, which accounts for 20% of group revenues, is now expected to improve profitability after being "pulled apart", O'Toole added.

FirstGroup's pre-tax profits rose 56% to £127.8m, boosted by a one-off decrease in pension liabilities related to the switch in pension increases from the RPI rate of inflation to CPI.